Magnetic resonance tomography (MR) requires ECG signals of the patient to be recorded in order to synchronize the triggering of MR measurement sequences to the heartbeat of the patient. The information relating to the current heart phase can likewise be obtained by the ECG signal recorded during an MR examination. If the ECG signals and the triggering and/or activation of the measurement sequence are not synchronized, there is a risk that the MR images contain movement artifacts.
In practice however, recording ECG signals is associated with difficulties, since the electrical and magnetic fields which take effect during the MR sequences are coupled into the ECG electronics system in a considerably interfering manner, thereby negatively affecting the reliable determination of the heart phase. Aside from these unwanted couplings into the ECG electronic system, the so-called magneto-hydrodynamic effect occurs in the case of higher magnetic flow densities, which result in an excessive rise in the T-wave of the heartbeat. In the field of electrocardiography, the different phases of the heart cycle are identified with letters, by the sequence P-Q-R-S-T for instance. In this process, the R-wave shows the greatest deflection, and is the reference point of the triggering and must consequently be determined in a reliable fashion.
A method for the ECG triggering of a measurement sequence of a magnetic resonance device is known from U.S. Pat. No. 6,070,097, but the ECG signals of a patient are recorded there by way of a single channel.
WO 99/04688 proposed the recording of two ECG channels, from which a vector representation in a coordinate system is derived. It should be possible to infer the R-wave of the heart cycle from this representation. It is however doubtful whether this method is sufficiently reliable, since this vector projection is dependent on many influences, for instance it changes if the patient holds his breath.
US 2004/0225210 A1 discloses an electrode arrangement, in which a number of lines are guided in parallel to each other.
DE 10 2005 004 859 A1 describes an ECG electrode arrangement for MR applications, in which each electrode line is assigned a correction line, which is insulated therefrom, in order to avoid unwanted induction voltages.
A typical conventional arrangement for recording ECG signals is shown schematically in FIG. 1. The arrangement includes three electrodes 1, which are each connected to a line 3 by way of clasps 2, the individual lines 3 are guided to an amplification unit 4, in which they are amplified for further processing purposes. The electrodes 1 are positioned on the thorax of a patient, it being possible for the cables 3 to be laid in any fashion. It is also possible to use more than three electrodes.
The recording of voltages U1 and U2 shown in FIG. 1 is carried out by way of two loops, which are each formed by two lines 3 and a path 5 in the body, shown with a dashed line. The ECG signal to be derived can be regarded as the voltage source U1 or UAVF.
A voltage for each loop can be measured on the amplification unit 4, said voltage being the sum of the derived ECG signal and an induced voltage part. The induced voltage part effects the interfering couplings into the ECG signal and can be calculated according to the law of induction. The surfaces S1 and S2 in this process correspond in each instance to the loop, which is formed by two lines and the path lying therebetween in the body of the patient. The larger this surface, the greater also the unwanted induced voltage.
If the ECG signals are recorded in order to trigger the measurement of a magnetic resonance device, the problem arises that the changing magnetic fields cause interferences to be coupled into the ECG signal. In the prior art, it has been proposed to filter out or suppress these interferences by means of a complex signal processing method. This procedure nevertheless requires a significant outlay for the signal processing.